/Engine/Source/Runtime/Experimental/Iris/Core/Public/Iris/Serialization/NetBitStreamWriter.h
Writes the BitCount least significant bits from Value. There's no validation of the Value, meaning it is allowed to contain garbage in the bits that are not going to be written to the buffer. If the BitCount exceeds the remaining space no bits will be written and the stream will be marked as overflown.
void WriteBits ( uint32 Value, uint32 BitCount )